This invention relates to cross-linked cellulose pulp sheets having low knot and nit levels and excellent absorbency and wet resiliency properties. More particularly, this invention relates to the cross-linking of cellulosic pulp fibers in sheet form and a method making cross-linked cellulose pulp sheets having performance properties which are equivalent or superior to those comprised of fibers which are cross-linked in fluff or individualized fiber form.
Within the specialty paper market there is a growing need for high porosity, high bulk, high absorbency pulps with superior wet resiliency. The filter, towel, and wipe industries particularly require a sheet or roll product having good porosity, absorbency and bulk, which is able to retain those properties even when wet pressed. A desirable sheet product should also have a permeability and/or absorbency which enables gas or liquid to readily pass through.
Pulps are cellulose products composed of cellulose fibers which, in turn, are composed of individual cellulose chains. Commonly, cellulose fibers are cross-linked in individualized form to impart advantageous properties such as increased absorbent capacity, bulk, and resilience to structures containing the cross-linked cellulose fibers.
Cross-linked cellulose fibers and methods for their preparation are widely known. Common cellulose cross-linking agents include aldehyde and urea-based formaldehyde addition products. See, for example, U.S. Pat. Nos. 3,224,926; 3,241,533; 3,932,209; 4,035,147; and 3,756,913. Because these commonly used cross-linkers, such as DMDHEU (dimethyloldihydroxy ethylene urea) or NMA (N-methylol acrylamide), can give rise to formaldehyde release, their applicability to absorbent products that contact human skin (e.g., diapers) has been limited by safety concerns. These cross-linkers are known to cause irritation to human skin. Moreover, formaldehyde, which persists in formaldehyde-cross-linked products, is a known health hazard and has been listed as a carcinogen by the EPA. To avoid formaldehyde release, carboxylic acids have been used for cross-linking. For example, European Patent Application EP 440,472 discloses utilizing carboxylic acids such as citric acid as wood pulp fiber cross-linkers.
For cross-linking cellulose pulp fibers, other polycarboxylic acids, i.e., C2-C9 polycarboxylic acids, specifically 1,2,3,4-butanetetracarboxylic (BCTA) or a 1,2,3-propane tricarboxylic acid, preferably citric acid, are described in EP 427, 317 and U.S. Pat. Nos. 5,183,707 and 5,190,563. U.S. Pat. No. 5,225,047 describes applying a debonding agent and a cross-linking agent of polycarboxylic acid, particularly BCTA, to slurried or sheeted cellulose fibers. Unlike citric acid, 1,2,3,4-butane tetracarboxylic acid is considered too expensive for use on a commercial scale.
Cross-linking with polyacrylic acids, is disclosed in U.S. Pat. No. 5,549,791 and WO 95/34710. Described therein is the use of a copolymer of acrylic acid and maleic acid with the acrylic acid monomeric unit predominating.
Generally, xe2x80x9ccuringxe2x80x9d refers to covalent bond formation (i.e., cross-link formation) between the cross-linking agent and the fiber. U.S. Pat. No. 5,755, 828 discloses using both a cross-linking agent and a polycarboxylic acid under partial curing conditions to provide cross-linked cellulose fibers having free pendent carboxylic acid groups. The free carboxylic acid groups improve the tensile properties of the resulting fibrous structures. The cross-linking agents include urea derivatives and maleic anhydride. The polycarboxylic acids include, e.g., acrylic acid polymers and polymaleic acid. Importantly, the cross-linking agent in U.S. Pat. No. 5,755,828 has a cure temperature, e.g., of about 165xc2x0 C. The cure temperature must be below the cure temperature of the polycarboxylic acids so that, through only partial curing, uncross-linked pendent carboxylic acid groups are provided. The treated pulp is defiberized and flash dried at the appropriate time and temperature for curing.
Intrafiber cross-linking and interfiber cross-linking have different applications. WO 98/30387 describes esterification and cross-linking of cellulosic cotton fibers or paper with maleic acid polymers for wrinkle resistance and wet strength. These properties are imparted by interfiber cross-linking. Interfiber cross-linking of cellulose fibers using homopolymers of maleic acid and terpolymers of maleic acid, acrylic acid and vinyl alcohol is described by Y. Xu, et al., in the Journal of the Technical Association of the Pulp and Paper Industry, TAPPI JOURNAL 81(11): 159-164 (1998). However, citric acid proved to be unsatisfactory for interfiber cross-linking. The failure of citric acid and the success of polymaleic acid in interfiber cross-linking shows that each class of polymeric carboxylic acids is unique and the potential of a compound or polymer to yield valuable attributes of commercial utility cannot be predicted. In U.S. Pat. No. 5,427,587, maleic acid containing polymers are similarly used to strengthen cellulose substrates. Rather than intrafiber cross-linking, this method involves interfiber ester cross-linking between cellulose molecules. Although polymers have been used to strengthen cellulosic material by interfiber cross-linking, interfiber cross-linking generally reduces absorbency.
Another material that acts as an interfiber cross-linker for wet strength applications, but performs poorly as a material for improving absorbency via intrafiber cross-linking is an aromatic polycarboxylic acid such as ethylene glycol bis(anhydrotrimellitate) resin described in WO 98/13545.
One material known to function in both applications (i.e., both interfiber cross-linking for improving wet-strength, and intrafiber cross-linking for improved absorbent and high bulk structures) is 1,2,3,4-butane tetracarboxylic acid. However, as mentioned above, it is presently too expensive to be utilized commercially.
Other pulps used for absorbent products included flash dried products such as those described in U.S. Pat. No. 5,695,486. This patent discloses a fibrous web of cellulose and cellulose acetate fibers treated with a chemical solvent and heat cured to bond the fibers. Pulp treated in this manner has high knot content and lacks the solvent resiliency and absorbent capacity of a cross-linked pulp.
Flash drying is unconstrained drying of pulps in a hot air stream. Flash drying and other mechanical treatments associated with flash drying can lead to the production of fines. Fines are shortened fibers, e.g., shorter than 0.2 mm, that will frequently cause dusting when the cross-linked product is used.
There are generally two different types of processes involved in treating and cross-linking pulps for various applications. In one approach, fibers are cross-linked with a cross-linking agent in individualized fiber form to promote intrafiber crosslinking. Another approach involves interfiber linking in sheet, board or pad form.
U.S. Pat. No. 5,998,511 discloses processes (and products derived therefrom) in which the fibers are cross-linked with polycarboxylic acids in individualized fiber form. After application of the crosslinking chemical, the cellulosic material is defiberized using various attrition devices so that it is in substantially individualized fibrous form prior to curing at elevated temperature (160-200xc2x0 C. for varying time periods) to promote cross-linking of the chemical and the cellulose fibers via intrafiber bonds rather then interfiber bonds.
This mechanical action has its advantages. In specialty paper applications, xe2x80x9cnitsxe2x80x9d are hard fiber bundles that do not come apart easily even when slurried in wet-laid operations. This process, in addition to promoting individualized fibers which minimize interfiber bonding during the subsequent curing step (which leads to undesirable xe2x80x9cnitsxe2x80x9d from the conventional paper pulps used in this technology), also promotes curling and twisting of the fibers which when cross-linked stiffens them and thereby results in more open absorbent structures which resist wet collapse and leads to improved performance (e.g., in absorbent and high porosity applications).
However, even when substantially well defibered prior to crosslinking, in specialty paper applications xe2x80x9cnitsxe2x80x9d can still be found in the finished product after blending with standard paper pulps to add porosity and bulk. When xe2x80x9cnitsxe2x80x9d are cross-linked in this form, they will not come apart.
Despite the advantages offered by the cross-linking approach in individualized form, many product applications (e.g., particularly in wet-laid specialty fiber applications) require undesirable xe2x80x9cnitsxe2x80x9d and xe2x80x9cknotsxe2x80x9d to be minimized as much as possible. Knots differ from xe2x80x9cnitsxe2x80x9d as they are fiber clumps that will generally not come apart in a dry-laid system, but will generally disperse in a wet laid system. Therefore, there is a need in the art to further minimize undesirable xe2x80x9cnitsxe2x80x9d and xe2x80x9cknotsxe2x80x9d.
Interfiber crosslinking in sheet, board or pad form, on the other hand, also has its place. In addition to its low processing cost, the PCT patent application WO 98/30387 describes esterification and interfiber crosslinking of paper pulp with polycarboxylic acid mixtures to improve wet strength. Interfiber cross-linking to impart wet strength to paper pulps using polycarboxylic acids has also been described by Y. Yu, et. al., (Tappi Journal, 81(11), 159 (1998), and in PCT patent application WO98/13545 where aromatic polycarboxylic acids were used.
Interfiber crosslinking in sheet, board or pad form normally produces very large quantities of xe2x80x9cknotsxe2x80x9d and xe2x80x9cnitsxe2x80x9d. Therefore, cross-linking a cellulosic structure in sheet form would be antithetical or contrary to the desired result, and indeed would be expected to maximize the potential for xe2x80x9cnitsxe2x80x9d and xe2x80x9cknotsxe2x80x9d resulting in poor performance in the desired applications.
Accordingly, there exists a need for an economical cross-linking process that produces cross-linked fibers that offer more superior wet strength and fewer xe2x80x9cknotsxe2x80x9d and xe2x80x9cnitsxe2x80x9d than current individualized cross-linking process. The present invention seeks to fulfill these needs and provides further related advantages.
In one aspect, this invention provides a method for preparing cross-linked cellulosic fibers in sheet form, the method comprising applying a cross-linking agent to a sheet of mercerized cellulosic fibers with a cellulose purity of at least about 90%, drying the cellulosic fiber sheet, and curing the cross-linking agent to form intrafiber rather than interfiber cross-links.
In another aspect, the present invention provides chemically cross-linked cellulosic fibers comprising mercerized cellulosic fibers in sheet form. In one embodiment, the polymeric carboxylic acid cross-linking agent is an acrylic acid polymer and, in another embodiment, the polymeric carboxylic acid cross-linking agent is a maleic acid polymer. In yet another embodiment, the present invention provides cross-linked cellulosic fibers comprising mercerized cellulosic fibers in sheet form cross-linked with a blend of polymeric carboxylic acid cross-linking agents and second cross-linking agent, preferably citric acid (a polycarboxylic acid).
Another aspect of the present invention provides a high bulk blended cellulose composition comprising a minor portion of mercerized high purity cellulose fibers which have been cross-linked with a polymeric carboxylic acid and a major proportion of uncross-linked cellulose fibers, such as standard paper grade pulps.
In yet another aspect, the present invention provides individualized, chemically cross-linked cellulosic fibers comprising high purity, mercerized individualized cellulosic fibers cross-linked with carboxylic acid cross-linking agents.
In still another aspect, the present invention provides absorbent structures that contain the sheeted, mercerized, high purity, carboxylic acid cross-linked fibers of this invention, and absorbent constructs incorporating such structures.
Advantageously, the invention economically provides cross-linked fibers having good bulking characteristics, good porosity and absorption, low fines, low nits, and low knots.
The present invention is directed to a method for forming chemically cross-linked cellulosic fibers with mercerized pulp in sheet form with carboxylic acid cross-linking agents. Preferably, the mercerized pulp is a high purity pulp. As used herein,: the term xe2x80x9chigh purityxe2x80x9d pulp refers to pulp with at least about 90% xcex1-cellulose content.
According to one embodiment, the mercerized cellulosic pulp fibers have an xcex1-cellulose content of at least about 90% by weight, preferably at least about 95% by weight, more preferably at least about 97% by weight, and even more preferably at least about 98% by weight.
Suitable purified mercerized cellulosic pulps would include, for example, Porosanier-J-HP, available from Rayonier Performance Fibers Division (Jesup, Ga.), and Buckeye""s HPZ products, available from Buckeye Technologies (Perry, Fla.). These mercerized softwood pulps have an alpha-cellulose purity of 95% or greater.
The cellulosic pulp fibers may be derived from a softwood pulp source with starting materials such as various pines (Southern pine, White pine, Caribbean pine), Western hemlock, various spruces, (e.g., Sitka Spruce), Douglas fir or mixture of same and/or from a hardwood pulp source with starting materials such as gum, maple, oak, eucalyptus, poplar, beech, or aspen or mixtures thereof.
Cross-linking agents suitable for use in the invention include homopolymers, copolymers and terpolymers, alone or in combination, prepared with maleic anhydride as the predominant monomer. Molecular weights can range from about 400 to about 100,000 preferably about 400 to about 4,000. The homopolymeric polymaleic acids contain the repeating maleic acid chemical unit xe2x80x94[CH(COOH)xe2x80x94CH(COOH)]nxe2x88x92, where n is 4 or more, preferably about 4 to about 40. In addition to maleic anhydride, maleic acid or fumaric acid may also be used.
As used herein, the term xe2x80x9cpolymeric carboxylic acidxe2x80x9d refers to a polymer having multiple carboxylic acid groups available for forming ester bonds with cellulose (i.e., crosslinks). Generally, the polymeric carboxylic acid crosslinking agents useful in the present invention are formed from monomers and/or comonomers that include carboxylic acid groups or functional groups that can be converted into carboxylic acid groups. Suitable crosslinking agents useful in forming the crosslinked fibers of the present invention include polyacrylic acid polymers, polymaleic acid polymers, copolymers of acrylic acid, copolymers of maleic acid, and mixtures thereof. Other suitable polymeric carboxylic acids include commercially available polycarboxylic acids such as polyaspartic, polyglutamic, poly(3-hydroxy)butyric acids, and polyitaconic acids. As used herein, the term xe2x80x9cpolyacrylic acid polymerxe2x80x9d refers to polymerized acrylic acid (i.e., polyacrylic acid); xe2x80x9ccopolymer of acrylic acidxe2x80x9d refers to a polymer formed from acrylic acid and a suitable comonomer, copolymers of acrylic acid and low molecular weight monoalkyl substituted phosphinates, phosphonates, and mixtures thereof; the term xe2x80x9cpolymaleic acid polymerxe2x80x9d refers to polymerized maleic acid (i.e., polymaleic acid) or maleic anhydride; and xe2x80x9ccopolymer of maleic acidxe2x80x9d refers to a polymer formed from maleic acid (or maleic anhydride) and a suitable comonomer, copolymers of maleic acid and low molecular weight monoalkyl substituted phosphinates, phosphonates, and mixtures thereof.
Polyacrylic acid polymers include polymers formed by polymerizing acrylic acid, acrylic acid esters, and mixtures thereof. Polymaleic acid polymers include polymers formed by polymerizing maleic acid, maleic acid esters, maleic anhydride, and mixtures thereof. Representative polyacrylic and polymaleic acid polymers are commercially available from Vinings Industries (Atlanta, Ga.) and BioLab Inc. (Decatur, Ga.).
Acceptable cross-linking agents of the invention are addition polymers prepared from at least one of maleic and fumaric acids, or the anhydrides thereof, alone or in combination with one or more other monomers copolymerized therewith, such as acrylic acid, methacrylic acid, crotonic acid, itaconic acid, aconitic acid (and their esters), acrylonitrile, acrylamide, vinyl acetate, styrene, a-methylstyrene, methyl vinyl ketone, vinyl alcohol, acrolein, ethylene and propylene. Polymaleic acid polymers (xe2x80x9cPMA polymersxe2x80x9d) useful in the present invention and methods of making the same are described, for example, in U.S. Pat. Nos. 3,810,834, 4,126,549, 5,427,587 and WO 98/30387. In a preferred embodiment, the PMA polymer is the hydrolysis product of a homopolymer of maleic anhydride. In other embodiments of the invention, the PMA polymer is a hydrolysis product derived from a copolymer of maleic anhydride and one of the monomers listed above. Another preferred PMA polymer is a terpolymer of maleic anhydride and two other monomers listed above. Maleic anhydride is the predominant monomer used in preparation of the preferred polymers. The molar ratio of maleic anhydride to the other monomers is typically in the range of about 2.5:1 to 9:1.
Preferably, the polymaleic acid polymers have the formula: 
wherein R1, and R2 independently are H, C1-C5 alkyl, substituted or unsubstituted, or aryl, and x and z are positive rational number or 0, y is a positive rational number and x+y+2=1; y is generally greater than 0.5, i.e. greater than 50% of the polymer. In many instances it is desired that y be less than 0.9, i.e. 90% of the polymer. A suitable range of y, therefore, is about 0.5 to about 0.9. Alkyl, as used herein, refers to saturated, unsaturated, branched and unbranched alkyls. Substituents on alkyl or elsewhere in the polymer include, but are not limited to carboxyl, hydroxy, alkoxy, amino, and alkylthiol substituents. Polymers of this type are described, for example, in WO 98/30387 which is herein incorporated by reference.
Polymaleic acid polymers suitable for use in the present invention have number average molecular weights of at least 400, and preferably from about 400 to about 100,000. Polymers having an average molecular weight from about 400 to about 4000 are more preferred in this invention, with an average molecular weight from about 600 to about 1400 most preferred. This contrasts with the preferred range of 40,000-1,000,000 for interfiber cross-linking of paper-type cellulosics to increase wet strength (see, e.g., WO 98/30387 of C. Yang, p. 7; and C. Yang, TAPPI JOURNAL).
Non-limiting examples of polymers suitable for use in the present invention include, e.g., a straight chain homopolymer of maleic acid, with at least 4 repeating units and a molecular weight, e.g., of at least 400; a terpolymer with maleic acid predominating, with molecular weight of at least 400.
In one embodiment, the present invention provides cellulose fibers that are cross-linked in sheet form with a blend of cross-linking agents that include the polymaleic and polyacrylic acids described herein, and a second cross-linking agent. Preferred second cross-linking agents include polycarboxylic acids, such as citric acid, tartaric acid, maleic acid, succinic acid, glutaric acid, citraconic acid, nialeic acid (and maleic anhydride), itaconic acid, and tartrate monosuccinic acid. In more preferred embodiments, the second cross-linking agent is citric acid or maleic acid (or maleic anhydride). Other preferred second cross-linking agents include glyoxal and glyoxylic acid.
A solution of the polymers is used to treat the cellulosic material. The solution is preferably aqueous. The solution includes carboxylic acids in an amount from about 2 weight percent to about 10 weight percent, preferably about 3.0 weight percent to about 6.0 weight percent. The solution has a pH preferably from about 1.5 to about 5.5, more preferably from about 2.5 to about 3.5.
The fibers, for example in sheeted or rolled form, preferably formed by wet laying in the conventional manner, are treated with the solution of crosslinking agent, e.g., by spraying, dipping, impregnation or other conventional application method so that the fibers are substantially uniformly saturated.
A cross-linking catalyst is applied before curing, preferably along with the carboxylic acids. Suitable catalysts for cross-linking include alkali metal salts of phosphorous containing acids such as alkali metal hypophosphites, alkali metal phosphites, alkali metal polyphosphonates, alkali metal phosphates, and alkali metal sulfonates. A particularly preferred catalyst is sodium hypophosphite. A suitable ratio of catalyst to carboxylic acids is, e.g., from 1:2 to 1:10, preferably 1:4 to 1:8.
Process conditions are also intended to decrease the formation of fines in the final product. In one embodiment, a sheet of wood pulp in a continuous roll form, is conveyed through a treatment zone where cross-linking agent is applied on one or both surfaces by conventional means such as spraying, rolling, dipping or other impregnation. The wet, treated pulp is then dried. It is then cured to effect cross-linking under appropriate thermal conditions, e.g., by heating to elevated temperatures for a time sufficient for curing, e.g. from about 175xc2x0 C. to about 200xc2x0 C., preferably about 185xc2x0 C. for a period of time of about 5 min. to about 30 min., preferably about 10 min. to about 20 min., most preferably about 15 min. Curing can be accomplished using a forced draft oven.
Drying and curing may be carried out, e.g., in hot gas streams such as air, inert gases, argon, nitrogen, etc. Air is most commonly used.
The cross-linked fibers can be characterized as having fluid retention values by GATS (Gravimetric Absorption Testing System) evaluation preferably of at least 9 g/g, more preferably at least 10 g/g, even more preferably at least 10.5 g/g or higher, and an absorption rate of at least 0.25 g/g/sec, more preferably at least 0.3 g/g/sec or higher than 0.3 g/g/sec. The cross-linked fibers also have good fluid acquisition time (i.e., fast fluid uptake).
Resulting cross-linked fibrous material prepared according to the invention can be used, e.g., as a bulking material, in high bulk specialty fiber applications which require good absorbency and porosity. The cross-linked fibers can be used, for example, in non-woven, fluff absorbent applications. The fibers can be used independently, or preferably incorporated into other cellulosic materials to form blends using conventional techniques. Air laid techniques are generally used to form absorbent products. In an air laid process, the fibers, alone or combined in blends with other fibers, are blown onto a forming screen. Wet laid processes may also be used, combining the cross-linked fibers of the invention with other cellulosic fibers to form sheets or webs of blends. Various final products can be made including acquisition layers or absorbent cores for diapers, feminine hygiene products, and other absorbent products such as meat pads or bandages; also filters, e.g., air laid filters containing 100% of the cross-linked fiber composition of the invention. Towels and wipes also, can be made with the fibers of the invention or blends thereof. Blends can contain a minor amount of the cross-linked fiber composition of the invention, e.g., from about 5% to about 40% by weight of the cross-linked composition of the invention, or less than 20 wt. %, preferably from about 5 wt. % to about 10 wt. % of the cross-linked composition of the invention, blended with a major amount, e.g., about 95 wt. % to about 60 wt. %, of uncross-linked wood pulp material or other cellulosic fibers, such as standard paper grade pulps.
There are several advantages in the present invention for cross-linking in sheet form besides being more economical. As noted above, cross-linking a cellulosic structure in sheet form would be expected to increase the potential for interfiber cross-linking which leads to xe2x80x9cnitsxe2x80x9d and xe2x80x9cknotsxe2x80x9d resulting in poor performance in the desired application. Thus, it was unexpected to find that high purity mercerized pulp cross-linked in sheet or board form actually yields far fewer xe2x80x9cknotsxe2x80x9d (xe2x80x9cnitsxe2x80x9d are a sub-component of the total xe2x80x9cknotxe2x80x9d content) than control pulps having conventional cellulose purity. When a standard purity fluff pulp, Rayfloc-J, was cross-linked in sheet form, the xe2x80x9cknotxe2x80x9d content went up substantially indicating increased deleterious interfiber bonding and examination of these xe2x80x9cknotsxe2x80x9d recovered by classification showed they contained true xe2x80x9cnitsxe2x80x9d (hard fiber bundles). Significantly, cross-linked pulp sheets according to the invention were found to contain far fewer knots than a commercial cross-linked pulp product of the Weyerhaeuser Company commonly referred to as HBA (for high-bulk additive) and a cross-linked pulp utilized in absorbent products by Proctor and Gamble (xe2x80x9cPandGxe2x80x9d), both of which are products cross-linked in xe2x80x9cindividualizedxe2x80x9d fibrous form using standard fluff pulps to minimize interfiber cross-linking.
When the cross-linked Porosanier sheeted pulps (prepared from wet laid pulp sheets using the preferred methodology described herein) were wet-blended with conventional paper pulp, Georgianier-J, at the 20% level to make handsheets for various tests to compare with handsheets similarly prepared using Weyerhaueser""s HBA, readily visible xe2x80x9cnitsxe2x80x9d were observed in the handsheets containing the HBA product, unlike those handsheets containing crosslinked Porosanier which were homogeneous in appearance with no visible xe2x80x9cnitsxe2x80x9d.
In diaper acquisition layer (AL) tests, where ability of the fibers to resist wet collapse upon multiple fluid insults (i.e., good wet resiliency) is important, it was observed that crosslinking of a conventional purity pulp (i.e., Rayfloc-J) in sheet form gave poor results compared to the commercial Proctor and Gamble AL material which is crosslinked with citric acid (the xe2x80x9cProctor and Gamble AL materialxe2x80x9d or the xe2x80x9cPandG AL materialxe2x80x9d). However, crosslinking of Porosanier-J-HP in sheet form gave much better results relative to Rayfloc-J. In fact, it was found that using high purity cellulose Porosanier sheets that are wet-laid in a non-homogeneous (or irregular manner) produced substantially better results than Porosanier sheets that are more uniform and homogeneous in nature. At equal basis weight, as well as average density levels, the Porosanier sheets are much softer and have areas in them that are more open as a result of more varied density throughout the dry sheet structure. The AL results on pads prepared from these cross-linked, non-homogeneous Porosanier sheets gave results that outperformed Proctor and Gamble citric acid cross-linked fibers on an overall basis, being about equal in acquisition times, but superior in rewet properties.
In another aspect of the invention, high purity mercerized pulp is cross-linked in individualized fibrous form using currently available approaches to obtain a product that is superior in acquisition time to those derived from conventional purity pulp used in current industrial practice. The rewet property, however, is poorer. The sheet treatment process of the instant invention offers an advantage of improved rewet properties.
Another benefit of using high purity cellulose pulp to produce cross-linked pulp or pulp sheet according to the invention is that because the color forming bodies are substantially removed (i.e., the hemicelluloses and lignins), the cellulose is more stable to color reversion at elevated temperature. Since polycarboxylic acid cross-liking of cellulose requires high temperatures (typically around 185xc2x0 C. for 10-15 minutes), this can lead to substantial discoloration with the conventional paper (or fluff) pulps that are presently used. In product applications where pulp brightness is an issue, the use of high purity cellulose pulp according to the invention offers additional advantages.
Another highly important benefit of the present invention is that cross-linked cellulose pulp sheets made in accordance with the invention enjoy the same or better performance characteristics as conventional individualized cross-linked cellulose fibers, but avoid the processing problems associated with dusty individualized cross-linked fibers.
To evaluate products obtained and described by the present disclosure as well as the invention herein, several tests were used to characterize cross-linked wood pulp product performance improvements resulting from the presently described method, and to describe some of the analytical properties of the products.